Dandruff and Seborrheic Dermatitis: A Head Scratcher 398 Role of Individual Susceptibility It is well known and often cited as a confounding fact that while Malassezia globosa is present on almost all humans only one-half to three-quarters of people suffer from dandruff. One hypothetical explanation of this phenomenon is the possibility that there exists a fundamental difference between dandruff suffers and non-dandruff individuals. To test this hypothesis we applied a fatty acid Malassezia metabolite, oleic acid, to the scalp of human volunteers who were clinically assessed as dandruff or non-dandruff. In this experiment (Figure 6), oleic acid dosed at a physiologically relevant concentration was able to induce a flaking response which was indistinguishable from dandruff by visual observation or electron microscopy in dandruff susceptible patients but not in non- susceptible patients.16 This finding provides evidence for a direct role of these fatty acid metabolites in dandruff and suggests an underlying difference amongst individuals that predisposes some to the development of dandruff or seborrheic dermatitis. The difference between dandruff susceptible and non-susceptible individuals remains unclear. Multiple possibilities exist, including innate differences in stratum corneum barrier function, skin permeability, and immune response to free fatty acids or proteins and polysaccharides from Malassezia. Further work will be necessary to fully understand the susceptibility response. It will be necessary to conduct significantly more research into Malassezia biology and its interaction with human skin to understand the fundamentals of the interactions. The sequencing of these genomes, in conjunction with the already sequenced human genome, will allow a detailed investigation of the metabolic interactions between human skin and Malassezia. As new pathways are elucidated, new intervention targets will arise. This new, groundbreaking research will enable development of new technologies to interrupt dandruff, which may not be dependent on and complimentary to existing antifungal treatments.